1. Field of the Invention
The present invention relates to a semiconductor device of an element using a ferroelectric thin film, particularly to a polarization inversion type non-volatile memory or a dynamic random access memory preferable to a large scale integrated circuit (LSI) and its fabrication method.
2. Description of the Prior Art
There are ferroelectric substances having an extremely large relative dielectric constant ranging from several hundreds to several thousands. Therefore, when a thin film made of these ferroelectric substances is used in a capacitor insulating film, there is provided a capacitor having a small area and a large capacitance preferable to a large scale integrated circuit (LSI). Further, a ferroelectric substance is provided with capacitor dielectric and its direction can be inverted by an outside electric field and accordingly, there is provided a non-volatile memory by using the characteristic.
There is disclosed a memory using a conventional ferroelectric substance in, for example, JP-A-5-90606. As shown by FIG. 22, a ferroelectric capacitor is formed by forming successively a lower Pt electrode 225, a ferroelectric thin film 226, an upper Pt electrode 227 and a Ti electrode 228 above an interlayer insulating film 224. Further, in the drawing, numeral 221 designates an isolation insulating film, numeral 222 designates a word line, numeral 223 designates an impurity diffused layer and numeral 229 designates an aluminum wiring layer. However, according to the technology, the respective layers are fabricated by independent masks and accordingly, there poses a problem of dimensional accuracy and matching accuracy. Hence, there has been proposed a structure disclosed in JP-A-2-288368. That is, as shown by FIG. 23, this is a method of subjecting an upper electrode layer 238, a ferroelectric film 237 and a lower electrode layer 236 summarizingly to dry etching. However, by the summarizing fabrication, leakage current is increased. Hence, there is a method disclosed in JP-A-3-256358 in which as shown by FIG. 24, only a lower electrode is fabricated and a ferroelectric film and an upper electrode are not fabricated for each cell but fabricated as a large pattern at an outer side of a memory mat or the like to thereby realize a highly integrated memory having a structure dispensing with matching allowance.
In the meantime, FIG. 25 shows a structure of another conventional memory cell disclosed in JP-A-7-14993. Although according to the structure, only a lower electrode is finely fabricated and a ferroelectric film and an upper electrode are not fabricated for each cell, there is a feature in that an adhesion layer 251 is interposed between an interlayer insulating film and a capacitor insulating film. It is described that as the adhesion layer, a layer of TiO2, ZrO2, Ta2O5, Si3N4 or the like is effective.
Further, as another conventional memory cell structure, there has been proposed a structure disclosed in JP-A-7-169854 in which as shown by FIG. 26, a lower electrode and a diffusion barrier layer are embedded in a reaction barrier film. This structure is obtained by the following process. First, an interlayer insulating film 248 and a polycrystal silicon film 246 are formed, thereafter, a titanium film 261 is formed, successively, a diffusion barrier layer 249 and a lower electrode 251 are formed. Thereafter, a ferroelectric film 252 is formed. In piling up the ferroelectric film 252, the titanium film is oxidized and the TiO2 film 261 of a reaction barrier layer is formed.
When lead zirconate titanate (PZT) is used in a capacitor insulating film in the above-described method disclosed in JP-A-3-256358, according to an investigation of the inventors, at a portion where PZT and a silicon oxide film which is an interlayer insulating film are brought into direct contact with each other, a reaction is caused therebetween. The reaction is caused even at low temperature of about 500xc2x0 C. and particularly when PZT is formed at temperatures equal to or higher than 700xc2x0 C., the silicon oxide layer completely reacts with PZT and a melted state is brought about. It becomes apparent that this phenomenon is caused by lead which is a major constituent element of PZT.
Further, in respect of the above-described method disclosed in JP-A-7-14993, according to an investigation of the inventors, it has been found that although Si3N4, in the adhesion layer reacts with PZT similar to the silicon oxide film, when TiO2, ZrO2, Ta2O5 is used for the adhesion layer, the adhesion layer serves as a reaction barrier layer between PZT and the silicon oxide film and therefore, the above-described problem of reaction between PZT and the silicon oxide film can be resolved. However, according to the structure, it has been clearly found that since side faces of a diffusion barrier layer 249 disposed below a lower electrode is exposed, when the PZT film is formed by CVD process or the like necessitating a heated oxidizing Ad atmosphere in the film forming operation, there poses a problem in which the diffusion barrier layer 249 is oxidized and the film is exfoliated. It has been found that even in the case of using sol-gel process, sputtering process, vapor deposition process or the like, there poses a similar problem in which in carrying out heat treatment of crystallization, the diffusion barrier layer 249 is oxidized. It seems that although a metal nitride of TiN, (Ti, Al)N, WN or the like is widely used in the diffusion barrier layer 249 and when the metal nitride is oxidized, nitrogen is discharged and therefore, exfoliation of the film formed thereon becomes significant.
In the meantime, according to the method disclosed in JP-A-7-169854, when Ti is oxidized, the volume is expanded and exfoliation of the ferroelectric film is brought about.
It is an object of the present invention to achieve a semiconductor device preventing reaction between a ferroelectric film and an insulating film and preventing film exfoliation and its fabrication method.
The above-described object is achieved by constituting a semiconductor device in which a reaction barrier film is provided between a ferroelectric film and an interlayer insulating film, side faces of the diffusion 2II barrier film and the ferroelectric film are not brought into contact with each other and side walls of a lower electrode and the ferroelectric film are brought into contact with each other.
By constructing the above-described constitution, in the case in which, for example, TiO2 is used for the reaction barrier film, when the film thickness is equal to or larger than 2 nm, it is effective in preventing a reaction between a silicon-species interlayer insulating film and lead included in a capacitor insulating film even in rapid heat treatment at about 700xc2x0 C. which is needed in crystallizing a PZT film. Further, only the diffusion barrier film is embedded into the reaction barrier film and therefore, the side walls of the lower electrode can be utilized as a capacitor, which is particularly effective in the case of applying to DRAM.
Further, the above-described object is achieved by embedding the diffusion barrier film in the interlayer insulating film as plugs and interposing the reaction barrier film between the capacitor insulating film and the interlayer insulating film. Also in this structure, a lower electrode is formed on the plug and accordingly, a ferroelectric film is installed from a side face to an upper face of the lower electrode, the side walls of the lower electrode can also be utilized as capacitor, which is particularly effective in the case of applying to DRAM.
Further, the above-described object is achieved by forming the reaction barrier film functioning to prevent reaction on the interlayer insulating film and thereafter forming the diffusion barrier film and the ferroelectric film. Before forming the diffusion barrier film and the ferroelectric film, the reaction barrier film is formed previously as an oxide and therefore, even when the reaction barrier film per se is formed by oxidizing a metal film, there poses no problem of exfoliation by volume expansion or the like.
Although there has been described of the case in which PZT is used for the capacitor insulating film, similar effect is observed even in the case of using lead-species ferroelectric substance other than PZT or Bi-species lamellar ferroelectric substance such as Bi4Ti3O12, Sr2Bi2Ta5O9 or the like. According to the Bi-species lamellar ferroelectric substance, generally, mutual diffusion with an interlayer insulating film becomes significant by an amount of the crystallizing temperature higher than that of the Pb-species ferroelectric substance and accordingly, the reaction barrier film becomes further necessary.
Further, it has been considered conventionally that in the case of dielectric substance which does not include lead or bismuth, for example, strontium barium titanate (BST), the reaction with the silicon oxide film constituting the matrix of the lower electrode is not significant and poses no serious problem. However, according to an investigation by the inventors, it has been found that although the diffusion coefficient is smaller than that of Pb or Bi, Ba or Sr also diffuses into the SiO2 matrix. Therefore, it is found that the significance of installing the reaction barrier film is great even in the case of using a film of BST-species.
The foregoing and other object, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in connection with the accompanying drawings.